1. Field of the Invention
The present invention relates to a receiver to be used for satellite broadcasting and satellite communication.
2. Description of the Prior Art
Hitherto, receivers for satellite communications were mainly for professional use, and composed of an exclusive receiver for each satellite, Receivers for satellite broadcasting were identical in modulation signal, and broadcasting was received by one receiver.
However, as the demand for satellite transmission increases and the band compression technology has advanced, methods for transmitting high definition signals in the same conventional exclusive bandwidth have been developed and put in practical use. Such examples are transmission systems represented by MUSE systems and MAC systems. As a result, signals differing in the modulation index from the conventional signals have come to mix in the conventional signals. In such situation, the existing receivers are receiving using the same demodulator.
As the number of satellites is increasing every year and broadcasting waves and communication waves are emitted from plural satellites, it is now necessary to select the satellite to receive depending on the purpose. In some of the satellites, the exclusive frequency bandwidth is different from each other, and in order to alter the bandwidth of the intermediate frequency filter in the prestage of the demodulator of the receiver, there were provided as many filters as the number of types of satellites differing in the exclusive bandwidth to be received.
In such conventional satellite broadcasting receivers, it is impossible to receive unless the circuits are added if the number of satellites increases in the future and the exclusive bandwidth varies or the frequency shift is different. If the frequency shift is different, for example, when receiving a signal of the MUSE system from the NTSC system, the amplitude of the demodulation signal differs and the gain of the video signal amplifier circuit must be changed. Or when the bandwidth of the filter is determined by adjusting to the signal of a large frequency shift, the noise electric power is larger as compared with the exclusive receiver adjusted to a signal of small frequency shift, and the sensitivity of the receiver is lowered relatively.